The grade(砂輪等級) of a bonded abrasive is a measure of its bond strength(膠合強度)• The grade is also referred to as the hardness (硬度) of a bonded abrasive• A hard wheel has a stronger bond and/or a larger amount of bonding material between the grains than a soft wheel• Structure (砂輪結構) of a bonded abrasive is a measure of its porosity (氣孔多寡